Configure your client software to talk to a new server, as usual. When asked for the server type, select 'GroupDAV' (some software may make mention of OpenGroupware.org servers, which are compatible with Citadel at the GroupDAV level). When asked for the path or URL, enter:

Special note for Kontact users: there is a known bug in some versions of Kontact in which the GroupDAV data will not be visible simply because you checked the box next to the resource. Instead, try right-clicking on the calendar or address book you want and enabling it from the context menu. (Thanks to Jon Watson for this tip.)